What are the differences between release procedure with classification and release procedure without classification? When are they used?

Release procedure with classification means the Purchase requsition can be released both at item level & at the header level. Release procedure with out classification can also be used for PR which is used for item level release only. all other external documents cannot be relesed with classification. The two procedures are mutually exclusive (that is to say, you must decide in favor of one of them only - you cannot use both).